Hydrophilic conjugated polymers are of growing interest as organic field effect transistors (OFETs), light-emitting diodes (OLEDs), photovoltaics (OPVs) and sensors. For example, conjugated polyelectrolytes (CPEs) and conjugated polyzwitterions (CPZs) integrate polar functionality pendent to conjugated aromatic backbones. Researchers have identified the utility of CPEs in optoelectronic and sensing applications and recent syntheses of CPZs show promise for producing polar, charge-neutral (counterion-free) electronically active materials for devices. (Hu, et al. Chem. Commun. 2015, 51, 5572-5585; Duarte, et al. Chem. Mater. 2011, 23 (3), 501-515; Jiang, et al. Angew. Chem. Int. Ed. 2009, 48 (24), 4300-4316; Duan, et al. Adv. Mater. 2011, 23, 1665-1669; Duan, et al. Chem. Sci. 2013, 4 (3), 1298-1307; Fang, et al. J. Am. Chem. Soc. 2011, 133 (4), 683-685; Kumar, et al. Energy Environ. Sci. 2013, 6 (5), 1589-1596; Page, et al. Macromolecules 2013, 46 (2), 344-351; Liu, et al. Adv. Mater. 2013, 25, 6868-6873; Page, et al. Chem. Sci. 2014, 5, 2368-2373; Page, et al. J. Polym. Sci., Part A Polym. Chem. 2015, 53 (2), 327-336.)
In poly(arylene-vinylene)s (PAVs), the vinylene linkages planarize the polymer backbone by removing torsional interactions between aryl rings, thus extending conjugation and tuning the band gap. Rotational flexibility about the vinyl group imparts solubility and solution processing. (Burroughes, et al. Nature 1990, 347 (11), 539-541; Grimsdale, et al. Chem. Rev. 2009, 109 (3), 897-1091; Feng, et al. Adv. Mater. 2008, 20 (14), 2684-2689.)
PAVs are currently synthesized from appropriate polymer precursors, or directly by polymerization of suitable monomers. The precursor routes begin by polymerization of quinodimethane monomers, followed by post-polymerization elimination to generate the conjugated structure. (Junkers, et al. Polym. Chem. 2012, 3 (2), 275-285.) These routes often generate structural defects, such as triple bonds and products of incomplete elimination. Direct routes to PAVs include transition-metal catalyzed polymerizations (i.e., Heck and Stille couplings), metathesis polymerizations, and transition-metal free polymerizations. (Buchmeiser Adv. Polym. Sci. 2005, 176, 89-119; Cho, et al. Adv. Mater. 1997, 9 (4), 326-328; Greenham, et al. Nature 1993, 365, 628-630; Pfeiffer, et al. Synt. Met. 1999, 101, 109-110; Pfeiffer, et al. Macromol. Chem. Phys. 1999, 200, 1870-1878; Synthesis and Properties of Poly(arylene vinylene)s, chapter 4. In: Handbook of Conducting Polymers. 3rd ed. Skotheim, T. A., Reynolds, J. R., Eds.; Taylor & Francis Group, LLC, 2007; pp 1-6.)
Horner-Wadsworth-Emmons (HWE) coupling represents a simple and effective approach to PAVs, giving reasonably high molecular weight and defect-free polymers, with a high degree of trans-olefins, without the need for metals or catalysts. Current PAV production by HWE coupling is typically performed in organic solvents using electron-rich monomers and strongly basic conditions. (Drury, et al. J. Mater. Chem. 2003, 13 (3), 485-490; Anuragudom, et al. Polym. Int. 2011, 60 (4), 660-665; Laughlin, et al. Macromolecules 2010, 43 (8), 3744-3749; Auragudom, et al. J. Polym. Res. 2009, 17 (3), 347-353; Anuragudom, et al. Macromolecules 2006, 39, 3494-3499; Davey, et al. Synt. Met. 1999, 103, 2478-2479.)
Inverted perovskite solar cells have gained widespread attention for their excellent compatibility with well-developed organic photovoltaic fabrication procedures. Replacing PEDOT:PSS with conjugated polyelectrolytes has afforded improved device performance and stability. (Meng, et al. Accounts. Chem. Res. 2015, 10.1021/acs.accounts.5b00404; Wu, et al. Energy & Environmental Science 2015, 8, 2725; Zuo, et al. Adv. Mater. 2014, 26, 6454; Malinkiewicz, et al. Nat. Photon. 2014, 8, 128; Etgar, et al. J. Am. Chem. Soc. 2012, 134, 17396; Liu, et al. Nature 2013, 501, 395; Jeng, et al. Adv. Mater. 2013, 25, 3727.)
In comparison with the fabrication of mesoscopic or titanium dioxide based planar perovskite solar cells that usually require high-temperature (above 450° C.) treatment, inverted perovskite solar cells are more compatible with well-established solution-based, low temperature, roll-to-roll fabrication procedures similarly used for the production of organic solar cells. (Song, et al. Journal of Materials Chemistry A 2015, 3, 9032; Docampo, P et al. Nat. Commun. 2013, 4; You, et al. ACS Nano 2014, 8, 1674.)
Recent progress in the optimization of perovskite film morphology on poly(3,4-ethylenedioxythiophene): poly(styrenesulfonic acid) (PEDOT:PSS) coated substrates has permitted inverted perovskite solar cells to achieve power conversion efficiencies (PCEs) >18%, which is approaching the record PCE of 20.1% obtained by their mesoscopic counterparts. (Heo, et al. Energy & Environmental Science 2015, 10.1039/C5EE00120J; Yang, et al. Science 2015, 348, 1234; Nie, et al. Science 2015, 347, 522.) Particularly, inverted perovskite/fullerene planar heterojunction solar cells have been shown to be effective at eliminating or suppressing the notorious photocurrent hysteresis often associated with perovskite solar cells. (Shao, et al. Nat. Commun. 2014, 5; Xu, et al. Nat. Commun. 2015, 6.)
In an inverted architecture, the perovskite film was deposited on top of a hole extraction layer (HEL), where PEDOT:PSS has been the primary material used. However, the acidic nature of PEDOT:PSS is detrimental to device performance and stability. Several solution processable inorganic materials, such as vanadium oxide (V2O5), nickel oxide (NiOX), and copper iodide (CuI), were developed as alternatives to PEDOT:PSS in inverted perovskite solar cells for efficient hole extraction. (Docampo, P et al. Nat. Commun. 2013, 4; Jørgensen, et al. Sol. Energ. Mat. Sol. C. 2008, 92, 686; Zhu, et al. Angewandte Chemie 2014, 126, 12779; Wang, et al. Scientific Reports 2014, 4, 4756; Yin, et al. Journal of Materials Chemistry A 2015, 10.1039/CSTA08193A; Chen, et al. Journal of Materials Chemistry A 2015, 3, 19353.) However, their processing methods are not as straightforward and simple as PEDOT:PSS. For example, V2O5 needs high temperature annealing, NiOX requires doping or high temperature annealing to afford high device performance, and organic volatile solvents (e.g., acetonitrile) are used to prepare CuI films.
Organic hole extraction materials have emerged as promising alternatives to PEDOT:PSS. (Malinkiewicz, et al. Nat. Photon. 2014, 8, 128; Malinkiewicz, et al. Adv. Energy Mater. 2014, 4, n/a; Lin, et al. Advanced Materials Interfaces 2015, 10.1002/admi.201500420n/a.) For example, Choi et al. reported a water/methanol-processed polyelectrolyte as a hole extraction material in inverted perovskite solar cells, only affording a maximum PCE of 12.5%. (Choi, et al. Nat. Commun. 2015, 6.) Li et al. developed a water-soluble polyelectrolyte for inverted perovskite solar cells, yet thermal annealing at 140° C. was a prerequisite HEL processing step. (Li, et al. Journal of Materials Chemistry A 2015, 3, 15024.) Although these organic hole extraction materials afford better device performance than their PEDOT:PSS counterpart, a full understanding of polyelectrolytes for hole extraction in inverted perovskite solar cells is lacking.
Accordingly, there remains an urgent, on-going need for novel hydrophilic conjugated polymers and improved hole-extraction materials, along with practical and efficient synthetic methods, that provide improved design of polymer-based solar cells and superior performances.